Compressors are one of several components in cooling and heating systems. They are an important component as the compressor is used to compress refrigerant gas used in the system, raising the pressure and the temperature of the gas. Depending on the system, the cycle can be reversed so that the compressor can be used to heat or cool a space. The compressor is typically used in combination with a condenser, expansion valves, an evaporator and blowers to heat or cool a space. Depending upon the direction of the cycle, the system can be used to remove heat from a preselected space or provide heat to a preselected space.
The compressor itself typically is a hermetically sealed device that has an intake port and a discharge port. The hermetically sealed device typically is a metallic shell that houses an electric motor and a mechanical means, such as an impeller or other mechanical portion, for compressing gas. For most compressor designs, the gas cavity enclosed by the housing serves as a reservoir of low-pressure gas to be drawn into the mechanical section of the compressor. The electric motor is connected to a power source that provides line power for operation. The motor in turn drives the means for compressing gas. Compressors are typically categorized by the means used to compress the gas. For example, compressors using a scroll compression device to compress refrigerant gas are referred to as scroll compressors; compressors using a piston device to compress the refrigerant gas are referred to as reciprocating compressors; compressors using rotating screw devices to compress a refrigerant gas are known as screw compressors. While there are differences among the compressors as to how refrigerant gas is compressed, the basic principles of operation as set forth above are common among the compressors, i.e. gas is drawn in through the gas intake when the motor is energized, the gas is compressed in the mechanical portion of the compressor and the highly compressed gas is discharged through an outlet port.
The variations among different compressor designs result in different noise generation mechanisms and overall different noise profiles. Different steps are taken to control or attenuate the sound in the different designs. Despite these efforts, there are common sources of noise for the various types of compressors. For example, a major source of noise can be found at the gas intake or suction port, where gas flow is regulated by a gas intake/suction valve mechanism. The gas intake/suction valve mechanism generates a high-level broadband sound. For hermetically sealed compressors, refrigerant is drawn from a cavity enclosed by the compressor housing into the gas compressing mechanism. During compressor operation, the sound is propagated upstream in the refrigerant gas stream and is radiated from the suction tube or tubes into the compressor's housing cavity. From there, the high level sound is transmitted from the housing cavity through the compressor housing shell and into the space surrounding the compressor. As can be seen, this sound is particularly undesirable when the compressor is located within, adjacent to or near a living area or a work area.
Of course, the sounds generated at the gas intake/suction valve mechanism are not new, and various methods have been attempted to eliminate, reduce or otherwise attenuate compressor noise. For example, it is well known that a foaming agent added to compressor oil will cause a reduction of sound within the compressor. It is believed that the foaming oil acts as an acoustic absorber. While this can be effective, the foaming oil must continue to perform under extremely taxing conditions, as it is exposed to refrigerant and to very high temperatures. The foam must not affect the lubricity of the oil and must not decompose as a result of interaction with the refrigerant and the high temperatures. Of course, if the foam deteriorates under these severe conditions, it loses its effectiveness as an acoustic attenuator. However, even when the foam does not deteriorate, since oil foam tends to be restricted to the bottom of the housing cavity, the foam is only partially effective in reducing the noise.
Other methods that have been utilized include mufflers. Mufflers are of two basic types, reactive mufflers and resistive mufflers. Reactive mufflers have been used to block sound at the suction tubes with limited success. Reactive mufflers are limited in their ability to reduce sound as their design makes them effective over a limited frequency range. These reactive mufflers sometimes utilize a resonator, or increase the length of flow of the gas by having it travel a tortuous path through openings of varying size. While they are effective within the designed frequency range, sound outside this frequency range is unaffected. While the sound energy created by the suction mechanisms of the compressor is broadband in character, the reactive mufflers only attenuate sound across a narrow range of frequencies. The remaining frequencies are propagated. The frequency bands that are propagated are referred to as band-pass frequencies. The designing of reactive mufflers for a predefined frequency region is difficult and even when successful, still does not block the broadband generated by the suction mechanism. Thus, the reactive mufflers tend to act as band-pass filters.
One example of a reactive muffler to muffle sound generated on the suction side of a compressor is set forth in U.S. Pat. No. 6,129,522 to Seo, issued Oct. 10, 2000. Sound is attenuated by passing inlet gas through a series of holes and openings of different sizes.
Resistive mufflers make use of a sound absorptive material to absorb sound over a wide range of frequencies. However, the materials typically used for sound absorbing purposes are not satisfactory choices for use in environments such as the high temperature, high flow velocity environments of refrigerant compressors, in which the materials are also exposed to chemicals such as compressor lubricants and refrigerants.
These resistive mufflers are located within the hermetic seal of the refrigerant compressor, and like other materials within the seal, are exposed to and saturated with lubricant and refrigerant, sometimes at temperatures in excess of 300° F. In addition, the high pressure fluctuations and associated pressure pulsations and vibrations also can adversely affect the sound absorptive materials. Not only is the acoustic performance of the sound insulation material significantly degraded when it is saturated with liquid, but also this harsh environment causes the material to fragment. Of course, the acoustic performance deteriorates as the sound insulation material disintegrates. However, what is more damaging is that the disintegrating material eventually mixes with the lubricating oil in the hermetically sealed compressor. Many insulation materials on dissociation can combine with typical refrigerants to form an acid. This acid can attack the metallic components of the compressor and the entire system. In addition, this material is deposited onto the moving parts with the lubricant. However, this material causes excessive wear and even binding of moving parts such as bearings. Because of this potential for failure of sound absorptive materials within the hermitically sealed compressor and the unsatisfactory results that accompany such failure, there has been a reluctance to incorporate resistive mufflers into refrigerant compressors. For example, polyurethane forms an open cell foam that is an effective acoustic absorber. However, in the harsh environment of a compressor, the cells collapse and the polyurethane combines with lubricants to form an undesirable, viscous fluid. Another effective acoustic absorber is solamide polyimide. But this material dissociates and causes deterioration of bearings.
What is needed is a muffler that absorbs sound over a broad range of frequencies. This is best accomplished by use of a resistive muffler. Therefore, what is needed is a resistive muffler that incorporates a sound insulation material that can survive the harsh environment of a compressor.